Need help making these functions using ONLY the following includes:

#include iostream #include string #include sstream #include fstream #include ioanip #include stdexcept #include limits #include cctype

using std::cin; using std::cout; using std::endl; using std::string; using std::getline; using std::ifstream; using std::invalid_argument

// PreCondition: a string that needs to be checked as a valid time // (non-negative float) // PostCondition: returns TRUE if string is only made up of numeric chars // 0-9 or '.' // - bool is_valid_time(const std::string& str) { }